* Clashes between rebels, pro-govt militias reported
* Western powers worry Libya will slide into chaos (Adds navy attack on tanker)
By Ulf Laessing and Ayman al-Warfalli
TRIPOLI/ES SIDER, Libya, March 11 (Reuters) - Libya's navy opened fire on Tuesday on a tanker carrying oil from a rebel-held eastern port and Italian vessels are helping to secure the now-stationary vessel, a Libyan military spokesman said.
Libya's parliament voted Prime Minister Ali Zeidan out of office earlier in the day after the tanker had escaped government forces, officials said.
Zeidan's removal deepens the turmoil in the North African country, a member of the OPEC oil producer group, where the government has struggled to rein in militias that helped oust Muammar Gaddafi in 2011 but now defy state authority.
Zeidan, a liberal weakened for months by infighting with Islamists, will be temporarily replaced by Defence Minister Abdallah al-Thinni, parliament spokesman Omar Hmeidan told reporters.
Deputies said they had mounted a no-confidence vote after the tanker managed to load crude at Es Sider port and moved into international waters.
The ship had sailed further east from Es Sider port before being attacked, Walid al-Tarhouni, spokesman for Libya's oil security force, told al-Nabaa television. He said the ship had initially escaped in bad weather.
Zeidan had told Reuters late on Monday in an interview the navy and allied militias had halted the tanker and were escorting it to a government-controlled port in western Libya.
He had for months been facing rising opposition from Islamists and the wider public, which blamed him for Libya's bumpy transition since the 2011 NATO-backed uprising.
"The government has been weakened for a time and we need a new personality," said al-Sharif al-Wafi, an independent lawmaker. A new prime minister will be elected by deputies within two weeks, he said.
On Saturday, Zeidan had said the navy would bomb the tanker if it tried to leave the port, one of three occupied by eastern rebels demanding a greater share of oil wealth and power.
In another worrying sign for the Tripoli government, rebel gunmen blocked the gate to an airbase in Sirte, a city located close to Es Sider.
Residents reported clashes in the afternoon between pro-government forces and rebels in the coastal city.
In Tripoli and the western city of Misrata, armed young men could be seen gathering to join a government force, residents said.
Western powers worry the conflict might break up the vast country dominated by competing tribes and militias, all heavily armed.
The port rebels also demand autonomy for the east, which had been neglected under Gaddafi as he concentrated power and wealth in Tripoli as well as his home region of Sirte.
The rebels, who have seized three ports and partly control a fourth, said they had dispatched forces to central Libya to deal with any government attack.
Libya has been trying to rebuild its army since Gaddafi's ousting, but analysts say it is not yet a match for battle-hardened militias that fought in the eight-month uprising that toppled the dictator. (Reporting by Ayman al-Warfalli in Es Sider, and Feras Bosalum and Ulf Laessing in Tripoli; Editing by Dale Hudson)